These research studies place primary emphasis on attempts to cultivate Mycobacterium leprae. As an approach to the problem, various tissue culture systems are used as substrates under different conditions for enhancing growth of the human leprosy bacilli. The attempts to grow the microorganism make use of a specilazed cell-impermeable diffusion chamber technique that was successfully developed in our laboratories for the cultivation of M. lepraemurium. Particular emphasis is placed on tissue cultures of established diploid and haploid cells derived from the frog as well as on a line mcarophage-like cells derived from a lymphoid neoplasm of the mouse. These cells offer the advantage for long-term maintenance for periods up to 120-150 days at a temperature of 25 degrees C and, hence, may be advantageous for growing M. leprae since it has such a long generation time. The tissue cultures are placed inside the chamber and then inoculated with M. leprae bacilli at a multiplicity of infection of 10 bacilli per cell within the chamber. The chambers then are maintained at 25 degrees C on monolayer petri plate cultures of the same type cells that are in the chamber. Yields of bacilli are determined at varying intervals and viability of the organisms are monitored by mouse footpad inoculations. Our studies indicate that this diffusion chamber system is a worthwhile model and technique for growth studies with M. leprae. The results show that the organisms replicate in chambers with various tissue culture substrates under properly controlled conditions and environment.